Molded articles based on pyrogenically prepared silicon dioxide, process for their production and their use
This application is a continuation of Ser. No. 308,125 filed Feb. 9, 1989 and now abandoned. 
    
    
     INTRODUCTION AND BACKGROUND 
     The present invention relates to molded articles based on pyrogenically prepared silicon dioxide, process for their production and their use as a catalyst support or catalyst. 
     Pyrogenically prepared silicon oxides are distinguished by being extremely finely divided and by a correspondingly high specific surface area, very high purity, a spherical particle shape and the lack of pores. Due to these properties, pyrogenically prepared oxides are becoming increasingly important as supports for catalysts; see D. Koth, H.. Ferch, Chem. Ing. Techn. 52, 628 (1980). 
     Since pyrogenically prepared oxides are particularly finely divided in nature, shaping of such powders to form catalyst supports or catalysts causes some difficulties. 
     German OLS 31 32 674 discloses a process for the production of molded articles from pyrogenically prepared oxides in which silica sol is uses as binder. 
     German OLS 34 06 185 discloses a process for the production of molded bodies by using glaze frit powder as a binder and glycerol as a lubricant. 
     German Patent 21 00 778 discloses employing granules based on pyrogenically prepared silicon dioxides as catalyst support used in processes for the preparation of, for example, vinyl acetate monomer. 
     These known processes have the disadvantage that the molded bodies so obtained do not have the optimum properties desired, such as, for example, high crushing strength, for certain catalytic reactions, i.g. the preparation of vinyl acetate from ethylene, acetic acid and oxygen or the hydration of ethylene to form ethanol. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide molded bodies based on pyrogenically prepared silicon dioxide and having the following physical-chemical characteristic data: 
     
         ______________________________________                                    
External diameter 2 to 15 mm                                              
BET surface area  50 to 400 m.sup.2 /g                                    
Total Pore volume 0.6 to 1.3 ml/g                                         
Crushing strength 40 to 120 N                                             
Pore size distribution                                                    
                  no pores &lt;5 nm in                                       
                  diameter, at least                                      
                  80% in the range 5                                      
                  to 40 nm in diameter                                    
Composition       &gt;95% by weight of SiO.sub.2                             
                  remainder Al.sub.2 O.sub.3.                             
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     Another object of the invention is to provide a process for the production of molded articles based on pyrogenically prepared silicon dioxide and having the following physical-chemical characteristic data: 
     
         ______________________________________                                    
External diameter 2 to 15 mm                                              
BET surface area  50 to 400 m.sup.2 /g                                    
Total pore volume 0.6 to 1.3 ml/g                                         
Crushing strength 40 to 120N                                              
Pore size distribution                                                    
                  no pores &lt;5 nm in                                       
                  diameter, at least                                      
                  80% in the range 5                                      
                  to 40 nm in diameter                                    
Composition       &gt;95% by weight of SiO.sub.2                             
                  remainder Al.sub.2 O.sub.3                              
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     which comprises homogenizing pyrogenically prepared silicon dioxide with kaolin and/or graphite, sugar, starch, urea or wax while adding water to the resulting mixture, drying the mixture so obtained at a temperature of from 80° to 120° C. and comminuting it to form a powder. Then the powder is pressed to form molded bodies or articles and the molded articles are tempered by heating at a temperature of from 400° to 1200° C. for a period of 0.5 to 6 hours. 
     In principle, all mixer apparatus or mills which enable good homogenization, such as, for example, paddle mixers, fluidized bed mixers, rotary mixers or airstream mixers, are suitable for carrying out the process according to the invention. Mixers by means of which it is possible to additionally compact the mixing material, for example plow blade mixers, chaser mills or ball mills, are particularly suitable. After homogenization, substantial drying at 80°-20° C. can be carried out so that a free-flowing powder is obtained after comminution. The molded articles can be produced by die stamping, eccentric pressing, extrusion or rotary pressing and in compactors. The shape of the molded articles can vary, for example, pellets, rods, or annular members e.g. rings. These shapes are well known in the field of catalytic supports. 
     In a particular embodiment of the invention, the mixture can have the following composition before pressing: 
     50-90% by weight of silicon dioxide, preferably 70-80% by weight 
     0.1-8% by weight of kaolin, preferably 1-5% by weight, and/or 
     0.1-10% by weight of graphite, preferably 1-5% by weight, 
     0.1-10% by weight of wax, preferably 1-5% by weight, 
     5-45% by weight of pore formers, such as urea, sugar or starch, preferably 10-30% by weight. 
     The molded bodies can have various shapes, for example cylindrical, spherical or annular, with an external diameter or from 2 to 15 mm. 
     Tempering of the molded articles is carried out by heating at 400°-1200° C. for 30 minutes to 6 hours. 
     By varying the amounts of starting materials and the pressing pressure, the crushing strength, the specific surface area and the pore volume can be modified within certain limits. 
     The molded articles prepared according to the invention can be used either directly as catalysts or as catalyst supports after the moldings have been impregnated, during or after their production, with a solution of a catalytically active substance and, if appropriate, have been activated by suitable after treatment. The compositions and procedures for deposition of catalytically active substances are well known in industry. 
     In particular, the molded articles made from pyrogenically prepared silicon dioxide can be used particularly well when used as the support for the catalyst used in the preparation of vinyl acetate monomer from ethylene, acetic acid and oxygen and as the catalyst in the hydration of ethylene. 
     The molded articles produced according to the invention have the following advantages: 
     High strength. 
     Large pore volume. 
     The majority of the pores are in the mesopore range. 
     No pores &lt;5 nm. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Examples 
     The pyrogenically prepared silicon dioxide is a silicon dioxide having the following physical-chemical characteristic data: 
     
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Silica (Aerosil)                                                          
            130  150   200  300   380                                     
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BET surface 130 ± 25                                                   
                 150 ± 15                                              
                       200 ± 25                                        
                            300 ± 30                                   
                                  380 ± 30                             
area m.sup.2 /g                                                           
Mean size of                                                              
            16   14    12   7     7                                       
the primary                                                               
particles nm                                                              
Compacted density.sup.1                                                   
            ca. 50                                                        
                 ca. 50                                                   
                       ca. 50                                             
                            ca. 50                                        
                                  ca. 50                                  
g/l                                                                       
Loss on drying.sup.2                                                      
            &lt;1.5 &lt;0.5.sup.7                                               
                       &lt;1.5 &lt;1.5  &lt;1.5                                    
(2 hours at 105° C.) %                                             
Ignition Loss.sup.2,5                                                     
            &lt;1   &lt;1    &lt;1   &lt;2    &lt;2.5                                    
(2 hours at 1000° C.) %                                            
pH.sup.3 (in 4%                                                           
            3.6-4.3                                                       
                 3.6-4.3                                                  
                       3.6-4.3                                            
                            3.6-4.3                                       
                                  3.6-4.3                                 
aqueous dispersion)                                                       
SiO.sub.2.sup.6 %                                                         
            &gt;99.8                                                         
                 &gt;99.8 &gt;99.8                                              
                            &gt;99.8 &gt;99.8                                   
A1.sub.2 O.sub.3.sup.6 %                                                  
            &lt;0.05                                                         
                 &lt;0.05 &lt;0.05                                              
                            &lt;0.05 &lt;0.05                                   
Fe.sub.2 O.sub.3.sup.6 %                                                  
            &lt;0.003                                                        
                 &lt;0.003                                                   
                       &lt;0.003                                             
                            &lt;0.003                                        
                                  &lt;0.003                                  
TiO.sub.2.sup.6 %                                                         
            &lt;0.03                                                         
                 &lt;0.03 &lt;0.03                                              
                            &lt;0.03 &lt;0.03                                   
HCl.sup.6,8 &lt;0.025                                                        
                 &lt;0.025                                                   
                       &lt;0.025                                             
                            &lt;0.025                                        
                                  &lt;0.025                                  
Sieving residue.sup.4                                                     
            &lt;0.5 &lt;0.5  &lt;0.5 &lt;0.5  &lt;0.5                                    
(by the method of                                                         
Mokker, 45 μm) %                                                       
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 .sup.1 in accordance with DIN 53 194                                     
 .sup.2 in accordance with DIN 55 921                                     
 .sup.3 in accordance with DIN 53 200                                     
 .sup.4 in accordance with DIN 53 580                                     
 .sup.5 based on the substance dried for 2 hours at 105                   
 .sup.6 based on the substance ignited for 2 hours at 1000° C.     
 .sup.7 -                                                                 
 .sup.8 HCl content is part of the ignition loss                          
 
    
     To prepare the silica(AEROSIL), a volatile silicon compound is atomized into an oxyhydrogen flame comprising hydrogen and air. In most cases, silicon tetrachloride is used. This substance hydrolyzes under the influence of the water produced in the oxyhydrogen gas reaction to form silicon dioxide and hydrochloric acid. After leaving the flame, the silicon dioxide enters a so-called coagulation zone, in which the silica(AEROSIL) primary particles and primary aggregates agglomerate. The product, which at this stage is in the form of a type of aerosol, is separated in cyclones from the gaseous accompanying substances and subsequently after treated with moist hot air. This process reduces the residual hydrochloric acid content to below 0.025%. Since, at the end of this process, the silica(AEROSIL) is produced with a bulk density of only about 15 g/l, the process is followed by a vacuum compression stage, by means of which compacted densities of about 50 g/l and more can be produced. 
     The particle sizes of the products obtained in this way can be varied using the reaction conditions. Such parameters are, for example, the flame temperature, the proportions of hydrogen and oxygen, the amount of silicon tetrachloride, the residence time in the flame or the length of the coagulation zone. 
     The BET surface area is determined in accordance with DIN 66 131 using nitrogen. 
     The pore volume is computed from the sum of the micropore, mesopore and macropore volumes. 
     The breaking strength is determined using a breaking tester from Ewerka Co., type TBH 28. 
     The micropores and mesopores are determined by recording an N 2  isotherm and evaluating same by the method BET, de Boer and Barret, Joyner, Halenda. 
     The macropores are determined by the Hg penetration method. Any suitable pyrogenically prepared silica can be used for purposes of the invention although the above silica(AEROSIL) is preferred. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Example 1 
     77% by weight of silica(Aerosil 200) 
     4% by weight of graphite 
     4% by weight of kaolin and 
     15% by weight of sugar are compacted with addition of water, dried at 100° C. for 24 hours, comminuted, to form a free-flowing product and pressed on an eccentric press to form pellets. 
     The crude pellets are tempered by heating at 900° C. for 4 hours. 
     The pellets obtained have the following physical-chemical characteristic data: 
     
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External diameter 9 mm                                                    
BET surface area  192 m.sup.2 /g                                          
Total pore volume 1.28 ml/g                                               
Breaking strength 45N                                                     
Pore size distribution                                                    
                  no pores &lt;5 nm in                                       
                  diameter, 81% of the                                    
                  pores in the range                                      
                  5 to 40 nm in diameter                                  
Composition       97.5% by weight of SiO.sub.2                            
                  2.5% by weight of Al.sub.2 O.sub.3                      
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     Example 2 
     77% by weight of silica(Aerosil 200) 
     4% by weight of graphite 
     4% by weight of kaolin and 
     15% by weight of starch and molded into pellets as in Example 1. 
     The pellets obtained have the following physical-chemical characteristic data: 
     
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External diameter                                                         
                 9 mm                                                     
BET surface area 195 m.sup.2 /g                                           
pore volume      1.16 ml/g                                                
Breaking strength                                                         
                 58N                                                      
Pore distribution                                                         
                 no pores &lt;5 nm in                                        
                 diameter, 81% of the                                     
                 pores in the range                                       
                 5 to 40 nm in diameter                                   
Composition      97.5% by weight of SiO.sub.2                             
                 2.5% by weight of Al.sub.2 O.sub.3                       
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     Example 2 
     77% by weight of silica(Aerosil 380) 
     19% by weight of urea 
     4% by weight of wax are molded to form pellets as in Example 1. 
     The tempering is carried out at 700° C. 
     The pellets obtained have the following physical-chemical characteristic data: 
     
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External diameter 5 mm                                                    
BET surface area  345 m.sup.2 /g                                          
Total pore volume 1.07 ml/g                                               
Breaking strength 42N                                                     
Pore size distribution                                                    
                  no pores &lt;5 nm in                                       
                  diameter, 88% of the                                    
                  pores in the range                                      
                  5 to 40 nm in diameter                                  
Composition       97.5% by weight of SiO.sub.2                            
                  2.5% by weight of Al.sub.2 O.sub.3                      
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     Further variations and modifications of the foregoing will be apparent to those skilled in the art from the foregoing and are intended to be encompassed by the appended claims. 
     German priority application P 38 03 895.1-45 is relied on and incorporated herein.